1. Field of the Invention
The invention relates to interlayer stress reduction. More specifically, the invention relates to use of membrane properties to reduce residual stress at an interlayer.
2. Background
Deposition layers in semiconductor processing have been shrinking since the invention of the microprocessor. Current thicknesses of deposition layers in state of the art microprocessors, for example, run to approximately 1 micron in depth. Any further reduction in feature size will make devices more prone to stress related reliability issues.
Whenever one material is deposited upon another, there is a mismatch between the materials that is inherent in the nature of the materials. This mismatch can be the result of differences in morphology, i.e., a crystal lattice, or this can be the result of differences in coefficient of thermal expansion between the two materials. Typically, when a material is deposited on another, the deposition takes place at an elevated temperature to enhance the ability of the deposited material to adhere to the surface of the material upon which it is deposited, i.e. the substrate. As these materials are cooled to operating temperatures, for example room temperature, if there is a mismatch in their coefficient of thermal expansion, one material will contract at a different rate from the other material. If the deposited material has a greater coefficient of thermal expansion than the substrate, the deposited material will contract more than the substrate area it covered at deposition. When the combination of the two materials reaches operating, or room temperature, there will be a tensile stress on the deposited material because of this difference in contraction. If, on the other hand, the coefficient of thermal expansion of the substrate material is greater than the deposited material, there will be a compressive stress on the deposited material due to the substrate""s shrinking faster than the deposited material.
Stress in an interlayer region such as this propagates into both layers of material; the deposited and the substrate material. This stress inhibits operation of the device in ways of, for example, degradation of electron mobility within the area of stress because the reciprocal lattice is no longer symmetrical. Additional results of high interfacial stress can be deteriorated electro-migration resistance, delamination at the interface, and stress migration where crystals move around to attain a relaxed state.
Electromigration is a failure phenomena that typically occurs when an electric current of high density is passed through a material. The end result is the formation of hillocks and voids in a material due to the development of severe mechanical stresses and stress gradients. Hillocks can lead to short circuit failures, and voids can lead to open circuit failures. Residual stress, for example like that formed in a mismatched interlayer region, may reduce the ability of an interlayer region to resist electromigration.
Delamination is a failure that generally takes place when the interlayer bond yields and the two materials separate. This happens, for example, when the differential in the morphological or crystallographic properties is sufficient to break the adhesion between the layers.
Stress migration is typically caused by stress mismatch between lattices. It results in atoms in the crystal lattice of either the deposited material, the substrate material, or both the deposited material and the substrate material moving around to attain a relaxed state. This movement may cause voids in an interface.
When depositing a relatively thin layer of a first material, or material xe2x80x9cAxe2x80x9d, on a second material, or material xe2x80x9cBxe2x80x9d, wherein the second material is acting as the substrate, typically the first material will become polycrystalline or amorphous as it deposits on the surface of the second material. This is because it is rare to have identical morphological, or crystallographic, properties in dissimilar materials. Whether morphological spacing of the material is smaller than the material that makes the substrate causing tension in the deposition layer, or the morphological structure of the deposition layer is larger than that of the substrate causing compression in the deposition layer, the material coming down on top of already deposited material xe2x80x9cAxe2x80x9d will see a confinement that is caused by the mismatch with material xe2x80x9cBxe2x80x9d. While the physical constants of material xe2x80x9cBxe2x80x9d are confining the deposition of material xe2x80x9cAxe2x80x9d, the growth is termed pseudomorphic, because material xe2x80x9cAxe2x80x9d is being constrained away from its own structure and toward the structure of material xe2x80x9cBxe2x80x9d. Once of course a sufficient critical thickness of material xe2x80x9cAxe2x80x9d has been reached, the deposited material relaxes and the growth is no longer pseudomorphic. However, prior to reaching this critical thickness, the deposited material is confined by the structure of the substrate material.